The present invention relates to an information read/write medium on which information can be recorded and from which information can be reproduced, and more particularly to the structure of the information read/write medium.
The present application claims priority from Japanese Patent Application No. 2002-188831, the disclosure of which is incorporated herein by reference.
In recent years, there is an increasing demand for a larger-storage-capacity information read/write medium (hereinafter referred to as an “optical disc”) which utilizes beams of light to record information thereon or reproduce information therefrom.
To meet this demand, research and development has been conducted on a phase change optical disc having two recording layers or a so-called two-layer recording phase-change optical disc, implementing a recording capacity being about twice as large as that of a phase change optical disc having only one recording layer.
FIG. 8 is a schematic sectional view illustrating a typical structure of this phase change optical disc, provided with a stack of layers “0” having a first recording layer 9 and a stack of layers “1” having a second recording layer 4.
The recording layers 4 and 9 are formed of a compound which is heated by a laser light beam or cooled to change its phase to either the crystalline phase or an amorphous phase, the phase change enabling information to be rewritten as many times as desired.
In this arrangement, the stack “1” is formed of a reflective layer 2, an enhanced layer 3, the aforementioned recording layer 4, and a protective layer 5, which are deposited sequentially in that order on a substrate 1.
On the other hand, the stack “0” is formed of a reflective layer 7, an enhanced layer 8, the aforementioned recording layer 9, and a protective layer 10, which are deposited sequentially in that order via an intermediate layer 6 overlying the protective layer 5.
The internal structure of the phase change optical disc is entirely covered and protected with the substrate 1 and a cover layer 11 deposited on the protective layer 10.
In the phase change optical disc constructed as described above, a laser beam enters the cover layer 11 at a predetermined power, thereby allowing information to be recorded on the disc or reproduced from the disc.
That is, to record information, a focused recording laser beam is incident upon the recording layer 9 in the stack “0,” thereby allowing information to be recorded onto the recording layer 9.
On the other hand, a focused recording laser beam directed to the recording layer 4 in the stack “1” passes through the stack “0” to impinge on the recording layer 4, thereby allowing information to be recorded onto the recording layer 4 by so-called direct overwriting.
In contrast to this, to reproduce information, a focused reproducing laser beam directed to the recording layer 9 in the stack “0” is reflected off the recording layer 9 or the like, and the stored information contained in the reflected beam is then subjected to signal processing or the like in a reproducing apparatus, thereby allowing information to be reproduced.
On the other hand, a focused reproducing laser beam directed to the second recording layer 4 in the stack “1” and having passed through the stack “0” is reflected off the recording layer 4 or the like, and the stored information contained in the reflected beam is then subjected to signal processing or the like in the reproducing apparatus, thereby allowing information to be reproduced.
As described above, attention has been focused on the storage medium of this type or the two-layer recording phase-change optical disc which comprises two recording layers for recording and reproducing information to provide an increased storage capacity, thereby enabling large volumes of data such as a long duration movie or high resolution image to be recorded or reproduced.
However, as can be seen from the structural property shown in FIG. 8, the aforementioned two-layer recording phase-change optical disc provides a small cooling effect to the recording layer 9 in the stack “0.”
For this reason, with this disc, there was a problem that a reproducing laser beam incident upon the recording layer 9 in the stack “0” caused the information already stored thereon to easily deteriorate since it was difficult for the heat produced by the absorbed laser beam to escape therefrom.
There was also another problem that due to the bad cooling effect upon direct overwriting on the recording layer 9, only a low recording bias power was allowed, thereby causing the information previously recorded thereon to be erased insufficiently.
Furthermore, the stack “1” is provided in close proximity to the substrate 1 and serves no function for transmitting laser beams. For example, this may allow a sufficiently thick metal layer having a good thermal conductivity to be formed between the stack “1” and the substrate 1 to provide an increased cooling effect to the recording layer 4, however, it is extremely difficult to form a similar metal layer having a sufficient thickness to allow the stack “0” to provide an increased cooling effect to the recording layer 4.
That is, suppose that the same thick metal layer as the one provided for the stack “1” is simply provided for the stack “0.” In this case, the incident laser beam would be blocked by the metal layer, thereby causing information to be recorded or reproduced with difficulty on or from the recording layer 4 in the stack “1.”
For this reason, the technique, which employs the aforementioned metal layer in an optical disc having only the stack “1” or the so-called one-layer recording phase-change optical disc to provide an increased cooling effect to the recording layer, can be used as it is with difficulty to cool the recording layer 9 in the stack “0” formed in the two-layer recording phase-change optical disc. Accordingly, it has been extremely critical to develop a novel technique which can overcome this difficulty.